The Silver Mirror experiment or Tollens' test is a test performed in the detection of aldehydes.
The experiment is named after its founder, German chemist Bernhard Tollens who did much work with carbohydrates and sugar structures which yielded his discovery of the Tollens reagent.
The Tollens' test proves structures of carbohydrates because it tests for reducing sugars and is able to then classify sugars like fructose as reducing sugars. It is used as a test for identifying aldehydes which can be oxidized to carboxylic acids.
An aldehyde is a functional group containing a carbonyl structure bonded to a hydrogen and an R group.
A carboxylic acid is a functional group which is quite similar in structure to an aldehyde, but has been oxidized; meaning it contains one more oxygen atom than the aldehyde. It contains a carbonyl group bonded to an R group, but also an alcohol or hydroxide compound as opposed to merely a hydrogen atom.
A brown precipitate of silver hydroxide forms in the reaction when the Ag+ and OH- ions are mixed with ammonia. This precipitate is dissolved when more ammonia is added which in turn forms a silver amine compound of [Ag(NH3)2]+.
This is a ball and stick diagram of the diaminesilver(I) cation:
This complex is broken down by glucose to the silver that we see as metallic, and as the glucose oxidizes it forms a gluconic acid. Below is a breakdown of this reaction:
The Tollens' reagent can not only identify aldehydes, but can also be used to test for certain ketones. An alpha-hydroxy ketone will react positively with the reagent.
A ketone is also much like an aldehyde. The main difference between the two is that a ketone is positioned in the middle of a particular carbon skeleton.
This is an alpha-hydroxy ketone, better known as an acyloin is an organic compound where the hydroxyl group is placed on the alpha position of the carbonyl group.
When adding any ketone other than acyloin, the Tollens' reagent will not yield any reaction. Ketones do not have a free hydrogen to react because of their structure, therefore making them very difficult to oxidize. Because of this feature, the Tollens' test does not prove useful for most ketones. The aldehydes contain the extra hydrogen that is able to oxidize which is why the test can prove an aldehyde and not a ketone.
Reducing sugars are determined by a solution called Fehling's reagent. A water soluble portion of the sample is reacted with copper sulfate and Fehling's solution (alkaline tartrate) under controlled temperature and concentration. The amount of copper reduced vs. the sugar in the analysis must remain proportional. Dextrose which is used in my adaptation of the experiment is considered a reducing sugar.
Everyday Uses of the Silver Mirror Chemicals:
In pathology, Tollens' reagent of ammoniacal silver nitrate is used in a silver stain technique used to detect melanin, and lipofuscin. These pigments are able to cause the reagent to turn to metallic silver.
Melanin is a pigment produced in the body which can cause freckles, red hair, and tanning in the skin.
Lipofuscin is an aging pigment found in the heart, liver, kidneys, muscle, and nerve cells.
Silver mirroring is used less frequently in modern times, but is still useful for restoring antique mirrors. When an old mirror is to be re-silvered, it is soaked in hydrochloric acid for about two days so the paint and silver layers flake off. Then the silvering process is performed on the glass.
The experiment is named after its founder, German chemist Bernhard Tollens who did much work with carbohydrates and sugar structures which yielded his discovery of the Tollens reagent.
The Tollens' test proves structures of carbohydrates because it tests for reducing sugars and is able to then classify sugars like fructose as reducing sugars. It is used as a test for identifying aldehydes which can be oxidized to carboxylic acids.
A brown precipitate of silver hydroxide forms in the reaction when the Ag+ and OH- ions are mixed with ammonia. This precipitate is dissolved when more ammonia is added which in turn forms a silver amine compound of [Ag(NH3)2]+.
This is a ball and stick diagram of the diaminesilver(I) cation:
This complex is broken down by glucose to the silver that we see as metallic, and as the glucose oxidizes it forms a gluconic acid.
Below is a breakdown of this reaction:
The Tollens' reagent can not only identify aldehydes, but can also be used to test for certain ketones. An alpha-hydroxy ketone will react positively with the reagent.
A ketone is also much like an aldehyde. The main difference between the two is that a ketone is positioned in the middle of a particular carbon skeleton.
This is an alpha-hydroxy ketone, better known as an acyloin is an organic compound where the hydroxyl group is placed on the alpha position of the carbonyl group.
When adding any ketone other than acyloin, the Tollens' reagent will not yield any reaction. Ketones do not have a free hydrogen to react because of their structure, therefore making them very difficult to oxidize. Because of this feature, the Tollens' test does not prove useful for most ketones. The aldehydes contain the extra hydrogen that is able to oxidize which is why the test can prove an aldehyde and not a ketone.
Reducing sugars are determined by a solution called Fehling's reagent. A water soluble portion of the sample is reacted with copper sulfate and Fehling's solution (alkaline tartrate) under controlled temperature and concentration. The amount of copper reduced vs. the sugar in the analysis must remain proportional. Dextrose which is used in my adaptation of the experiment is considered a reducing sugar.
Everyday Uses of the Silver Mirror Chemicals:
In pathology, Tollens' reagent of ammoniacal silver nitrate is used in a silver stain technique used to detect melanin, and lipofuscin. These pigments are able to cause the reagent to turn to metallic silver.
Melanin is a pigment produced in the body which can cause freckles, red hair, and tanning in the skin.
Lipofuscin is an aging pigment found in the heart, liver, kidneys, muscle, and nerve cells.
Silver mirroring is used less frequently in modern times, but is still useful for restoring antique mirrors. When an old mirror is to be re-silvered, it is soaked in hydrochloric acid for about two days so the paint and silver layers flake off. Then the silvering process is performed on the glass.